Additive manufacturing (AM) is the process of producing a three-dimensional structure by building up successive layers based on a set of digital instructions. Selective laser sintering is a known method of AM which typically includes the following general steps. Powdered material such as plastic, metal, ceramic or glass is spread onto a platform and a laser is used to selectively heat an area of powdered material corresponding to a two-dimensional cross section of the final three dimensional structure. The heat from the laser causes the powdered material to fuse together. The platform is lowered and a new layer of powder is applied. The laser then selectively heats an area of powdered material corresponding to the next cross section fusing this to the cross section below. This process is repeated until the three-dimensional structure is completed. One application of additive manufacturing is for the production of porous titanium bone scaffolds for the customised medical implant market.
Traditionally, bone grafts have been used to repair damaged bone. More recently, materials such as titanium and titanium alloys have been used to produce load-bearing implants. However, titanium implants suffer from various problems. A particular problem is the stiffness of titanium relative to bone. Using Young's Modulus scale, titanium has a stiffness of 125 Gpa whereas bone has a stiffness of just 20 Gpa. Use of a titanium implant can therefore lead to “stress-shielding”, meaning that the titanium implant takes the strain of weight and pressure leaving surrounding bone to weaken. This can also lead to implant loosening which is described in Wen, Cuie “Report: New titanium alloys and scaffolds with ideal biocompatibility for biomedical applications” Swinburne University of Technology, Published 2013 Nov. 18, which is herein incorporated by reference in its entirety. It is therefore desirable for bone scaffolds to have a stiffness closer to that of bone to limit or eliminate stress-shielding.
In addition, studies have shown that the release of metal ions from implant materials might have adverse biological effect or elicit allergy reaction. A current solution to this problem is careful selection of the composition of metal biomaterials to avoid or minimise adverse.
An improvement on dense metallic implants has been the use of porous scaffolds which mimic the structure of bone and allow bone tissue ingrowth, in a process known as osseointegration. Such scaffolds can be produced using “space holder” additive manufacturing methods, whereby AM is used to form a three dimensional structure comprising dispersed filler particles, and subsequently the filler particles are degraded to leave behind a porous structure. Porous structures can also be produced by foaming titanium alloy (see http://biometal.sjtu.edu.cn/en/Show.aspx?info lb=517&info id=784&flag=293).
Pyrolytic carbon or pyrocarbon is a synthetic substance that is generally produced by heating organic material in the absence of oxygen. It has excellent biocompatibility and hardness and is anti-thrombotic. This has led to the use of pyrolytic carbon in the production of small orthopaedic, dental and maxillofacial implants such as proximal interphalangeal (PIP) joints and bone plates.